1. Field of the Invention
The present invention generally relates to printing devices in which latent images are developed by means of a two-component developer, and more particularly to a toner supply control system and method for optically detecting the density of a developed latent image of a toner mark and controlling toner supply on the basis of the detected density.
2. Description of the Prior Art
Printing devices are widely used for computers, copying machines, facsimiles and the like. Examples of the printing devices are an electrophotographic printing device and an electrostatic recording device. In such printing devices, latent images are developed by a developer and thereby visual images are formed.
Many printing devices employ a developing process using a two-component developer consisting of a carrier and toner. The developing process consumes toner. Hence, the printing devices are equipped with toner supply units, which supply developing units with toner as necessary.
A toner supply control optically detects the density of a developed latent image of a toner mark, and controls the toner supply on the basis of the detected density.
FIGS. 1, 2A through 2D show an electrophotographic printing device that employs a toner supply control as described above.
Referring to FIG. 1, the printing device comprises a photosensitive drum 10 around which there are provided a corona charger 11, an optical image writing unit 12, a developing unit 13 with a toner supply unit 14, an image transfer unit 15, an AC discharging unit 16, a cleaning unit 17, and an optical density sensor 18. The optical image writing unit 12 includes a light source formed with, for example, a semiconductor laser, a polygonal mirror (optical scanning device), and a mirror which causes a scanning light emitted from the light source to be projected onto the photosensitive drum 10. The developing unit 13 comprises a magnetic roller 130, a supply roller 131, a stirring member 132, and a flow plate 133. The magnetic roller 130 supplies the two-component developer consisting of carriers and toner particles. The supply roller 131 supplies the magnetic roller 130 with the two-component developer. The stirring member 132 stirs and mixes the carriers and toner particles. The flow plate 133 guides the developer remaining on the magnetic roller 130 to the stirring member 132.
The optical density sensor 18 is made up of a light-emitting element (LED) 18a and a light receiving element 18b, as shown in FIG. 2A. As shown in FIG. 2A, a light from the light-emitting element 18a is projected onto a developed image of a toner mark M (FIG. 2B) formed outside a printing area on the photosensitive drum 10. The light receiving element receives the light reflected by the developed image of the toner mark M, and generates a detection voltage dependent on the density of the developed image of the toner mark M.
The electrophotographic printing device operates as follows. The photosensitive drum 10 is uniformly charged by the corona charger 11. The photosensitive drum 10 being rotated is linearly scanned by a light emitted from the optical image writing unit 12 and modulated by a video signal. In this manner, electrostatic latent images are formed on the photosensitive drum 10. The latent images are developed by the developing unit 13. Then, toner images generated by developing are transferred to a sheet transported in a leftward direction by means of transport rollers 30. Thereafter, the sheet is detached from the photosensitive drum 10 by the AC discharging unit 16.
The photosensitive drum 10, after the image transfer process, is subjected to a cleaning process in which remaining toner particles are removed by means of a cleaning brush and a blade of the cleaning unit 17. Then, the sheet is sent to an image fixing unit, in which the images on the sheet are fixed thereon.
The latent image of the toner mark M is formed, under the control of a print controller 2, on the photosensitive drum 10 via the optical image writing unit 12 once per revolution of the photosensitive drum 10. The latent image of the toner mark M is developed in the same manner as the images formed within the image forming area. The sensor 18 optically reads the developed image of the toner mark M and generates a detection voltage dependent on the density of the developed image of the toner mark M.
When the toner mark M has a pattern shown in FIG. 2B, a sensor output (volts) vs. toner density (weight percent) shown in FIG. 2C is obtained. The sensor output obtained with the toner density equal to 4.0 wt % is defined as a slice or reference level, which is stored in the print controller 2. The output signal of the light-receiving element 18b of the optical density sensor 18 is compared with the reference level in the print controller 2. When the output signal of the sensor 18 is less than or equal to the reference level, it is determined that the toner density is low. In this case, the print controller 2 drives the supply motor 143 in order to rotate the supply roller 142, so that toner is supplied to the developing unit 13 from a toner reservoir 140 of the toner supply unit 14. In this manner, the toner density is regulated at a fixed level.
As is known, the two-component developer deteriorates in the progress of use thereof, and the developing characteristic thereof changes. This mainly results from deterioration of carrier particles, particularly, surface variations thereof. For example, when the two-component developer is used for a long time, toner particles adheres to the surfaces of carrier particles because of friction between the toner particles and the carrier particles. This increases the electric resistance of the carrier particles and changes the developing characteristic. Hence, the print image density becomes low.
The above phenomenon causes the following problems. The print image density of the toner mark M decreases as the number of printed sheets increases even when the toner density is maintained at a constant level. Further, the output signal of the sensor 18 decreases and the slope of the sensor output signal vs. toner density characteristic becomes small as the number of printed sheets increases, as shown in FIG. 2C.
The change in the characteristic shown in FIG. 2C causes the following problems. In the case where toner is supplied so that the sensor output level is maintained at the reference level, the developing characteristic of the developer varies and hence the print image density tends to decrease even for the same toner density as the number of printed sheets increases. Hence, the toner mark density detected by the sensor 18 does not increase in proportion as the toner density increases. Hence, as shown in FIG. 2D, the toner density continues to increase, and the toner mark density does not correctly reflect the toner density.
There is an upper limit regarding the toner density. As the toner density increases, sufficient stirring cannot be carried out. This increases uncharged toner particles and noise will appear on the background of images. Further, toner particles are liable to be scattered because of uncharged toner particles and the inside of the device is contaminated.
If a sensor output lower than the reference level shown in FIG. 2C is defined as the reference level in order to suppress an increase in the toner density and prevent occurrence of uncharged toner particles, the print image density decreases and the detection sensitivity of the toner density decreases (the slope of the sensor output vs. toner density characteristic becomes small). As a result, the range within which the sensor output varies in response to a decrease in the print image density becomes narrow, and hence the toner supply control cannot be performed with high precision.